Avoid using glibc internals for SIGSYS handler

SIGSYS fields were added to siginfo_t on glibc in version 2.17,
which is quite old now. Additionally, musl libc uses a different
name (__si_fields) for its internal struct members.

To fix this, use the libc-defined members if they are available,
otherwise fall back to _sifields, as is done currently.

Change-Id: I6a542a1817df7e31309a1043b899328a50834f7d
Bug: 1045069
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015735
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734970}

• sandbox/linux/seccomp-bpf/trap.cc[diff]